Flexible filament cutting elements often become worn, frayed or broken as a result of normal usage. Their effective cutting lengths are thereby shortened. Therefore, it is necessary to provide apparatus for uncoiling, playing out or otherwise providing a new length of flexible filament cutting element to replace the lengths which become unusable under normal operating conditions. The prime mover for the cutting devices can be either a light weight electric motor or gasoline engine. Excessive flexible filament cutting element length increases the probability of electric motor burn-out when such as electric motor is utilized. Thus, it is desirable to provide an apparatus which permits the play-out of the flexible cutting element to predetermined discrete increments and which furthermore maintains the flexible filament cutting element length less than a predetermined maximum.
Devices are known in the prior art for metering a length of the flexible filament cutting element. In one such device, a flexible cutting line is wound about a spool. The spool has a square recess and is spring biased into a position where it engages a square-headed bolt secured to a motor shaft. When it is necessary to uncoil additional cutting line, the device is de-energized and allowed to stop rotating. Then, the spool is manually depressed against the biasing force of the spring to disengage the bolt and recess. The spool is then free to turn. Upon release, it may again engage the bolt head. The operator rotates the spool until he judges that the necessary additional length of flexible line has been removed from the coil.
Another filament line metering device is disclosed in a patent application of Henry B. Tilliston which is entitled "Line Metering Apparatus", filed on Aug. 13, 1976 under Ser. No. 714,013 and assigned to the same corporation to which this application is assigned. The apparatus disclosed therein meters discrete lengths of filament from a filament holder or spool in an edging and trimming device.
The line metering is specifically accomplished in the aforesaid Tilliston application through the use of a spool on which the filament is wound, and which is mounted for both axial and rotational movement relative to the rotating drive shaft. The drive shaft carries a drive gear having teeth which project radially outward. The filament holder carries a driven gear having first and second sets of gear teeth which project radially inward for selective engagement with the drive gear teeth. The two sets of driven gear teeth are spaced axially from each other and rotationally staggered a predetermined angular amount. The filament holder is normally axially biased so that the drive gear teeth engage the upper set of driven gear teeth.
When the free filament end becomes worn or broken, additional filament may be metered by tapping the lower surface of the filament holder on the ground, which moves the filament holder upward axially against the bias of the spring until the drive gear teeth move out of engagement with the first set of driven gear teeth and into engagement with the second set of driven gear teeth. As this happens, the staggered relation of the driven gear teeth sets causes limited relative rotational movement of the spool relative to the drive shaft, which causes a predetermined length of filament to unwind. After the tapping force is removed, the spool returns to its original state and an additional length of filament unwinds in the same manner. The line metering mechanism disclosed in the Tilliston application is particularly suitable for use in an edging and trimming device which is generally intended to be carried and which therefore lends itself to the "tapping" approach for filament metering. Such a "tapping" approach, however, is not particularly suitable for line filament lawn mowers. This is true because rotary lawn mowers are generally wheeled, which of course is intended to preclude elevational movement of the lawn mower relative to the ground during operation. Thus, axial movement of a filament spool from the bottom of the lawn mower to effect filament metering during the cutting operation is not feasible.
Another line metering device, which is particularly suitable for use in rotary line filament lawn mowers is disclosed in the patent application Ser. No. 821,145 of James R. Bowman et al which is entitled "Line Feed Mechanism for Filament Mower", filed on Aug. 2, 1977, and assigned to the same corporation to which this application is assigned.
The line metering apparatus disclosed in the Bowman et al application accomplishes the line metering by maintaining the filament spool in an axial stationary position while at the same time mounting the spool for rotation relative to a drive member. The drive member is rotatably driven and axially movable between first and second axial positions. In each axial position the drive member is in driving engagement with a driven member secured to the spool. The drive and driven members are constructed so that, as the drive member moves from one axial position to the other, the driven member rotates a limited amount relative to the drive member in a direction which permits filament to centrifugally unwind from the spool.
Metering is thus accomplished by axially moving the drive member between its axial positions relative to the driven member and spool. A means is provided for affecting axial movement of the drive member as it rotates. This is accomplished through means for mounting having the drive member to the drive shaft so that the drive member always rotates with the drive shaft but is axially moveable thereto. An actuating member, preferably taking the form of an indexing sleeve, is mounted in opposition to the rotating drive member. The actuating member is essentially stationary from the standpoint of rotation, but is axially movable through camming structure which is manually controlled by the lawn mower operator. Rotatable bearing means are disposed between the actuating member and drive member, permitting axial shifting to occur even though the drive member rotates at substantial rotational velocity relative to the actuating member.